The present invention relates to an electric power steering apparatus.
Conventionally, electric power steering apparatuses (hereinafter, referred to as EPS) have been known that apply force of a motor to the steering system of a vehicle to assist steering operation of the driver. For example, Japanese Laid-Open Patent Publication No. 2012-25246 discloses an EPS that includes a rack-and-pinion mechanism as a steering system. The rack-and-pinion mechanism includes a pinion, which rotates according to a steering operation, and a rack shaft, which meshes with the pinion. The rack-and-pinion mechanism converts rotation of the pinion to linear motion of the rack shaft, thereby changing the direction of wheels. The rack shaft includes a ball screw mechanism, which converts rotation of a motor to linear motion of the rack shaft. That is, the rotational force of the motor is used to assist the motion of the rack shaft, which in turn assists steering operations.
Specifically, as shown in FIG. 6, an EPS 100 transmits rotational force of a motor 101 to a rack shaft 106 via a cylindrical driver pulley 102, a timing belt 103, a cylindrical driven pulley 104, and ball screw mechanism 105. The ends of the rack shaft 106 are each coupled to a wheel via a ball joint.
The ball screw mechanism 105 includes a ball screw portion 105a, which is formed in part of the rack shaft 106, and a ball nut 107, which meshes with the ball screw portion 105a via balls. The ball nut 107 has a flange 107a, which is formed on the outer circumferential surface at a first end 107d, and a male thread 107b, which is formed on the outer circumferential surface at a second end 107e. A lock nut 108 is fastened to the male thread 107b. 
The driven pulley 104 and a ball bearing 109 are arranged along the outer circumferential surface of the ball nut 107 and between the flange 107a and the lock nut 108. The driven pulley 104 is arranged to contact the flange 107a, and the ball bearing 109 is arranged to contact the lock nut 108. The driven pulley 104 and the ball bearing 109 (the inner race of the ball bearing 109, to be precise) are brought into an area contact with each other at a position between the driven pulley 104 and the ball bearing 109 with respect to the axial direction of the ball nut 107.
The driven pulley 104 has an annular step 104a, which is located on the inner circumferential surface at an end separated away from the ball bearing 109. The step 104a is formed at the end of the driven pulley 104 that is separated away from the ball bearing 109 by increasing the inner diameter of the driven pulley 104 without increasing the outer diameter thereof. The step 104a contacts the flange 107a in the axial direction of the ball nut 107.
The driven pulley 104 and the ball bearing 109 (the inner race) are clamped between the flange 107a of the ball nut 107 and the lock nut 108, which is fastened to the male thread 107b. Accordingly, the driven pulley 104 and the ball bearing 109 are restricted from moving in the axial direction relative to the ball nut 107. The driven pulley 104 and the ball bearing 109 are fixed to the ball nut 107 so that the ball nut 107 rotates integrally with the driven pulley 104.
In recent years, the energy saving aspect of EPS has been receiving attention, and more and more models of cars have been equipped with an EPS like that disclosed in Japanese Laid-Open Patent Publication No. 2012-25246. On the other hand, demands for reduction in the weight and costs of EPS have become more and more intense. To meet such demands, an EPS having a reduced number of components and a simple structure has been desired.